1. Field of the Invention
The present invention relates to a Stirling engine. More particularly, the present invention relates to a solar-powered Stirling engine and a method for controlling the output of such an engine.
2. Description of Related Art
U.S. Pat. No. 4,457,133 discloses a conventional solar-powered Stirling engine that includes a plurality of cylinders. In each cylinder, a piston is movably mounted and an expansion space and a compression space are defined across the piston. The compression space is in fluid communication with a neighboring expansion space via a heater, a regenerator and a cooler. Reciprocal movements of each piston are converted into a rotating torque at an output mechanism and the resulting torque is transmitted to a suitable mechanism such as a dynamo.
A working gas such as helium gas or hydrogen gas is filled in a working space that includes the expansion space, the compression space and the aforementioned other members located therebetween, and the gas is heated by solar radiation energy while it passes through pipes. The working space is connected to a gas-reservoir via a minimum-cycle-pressure line which includes a check-valve and a pressure-increasing valve. The working space is also connected to the gas reservoir via a maximum-pressure-line which includes a check valve, a pressure-decreasing valve and a compressor. As a result of that construction, when the pressure-increasing valve is opened, the average pressure is increased and the engine output is increased. Alternatively, when the pressure-decreasing valve is opened, the average pressure is decreased and the engine output is decreased.
In the conventional Stirling engine, the engine-output is controlled as follows:
(1) Since a temperature T of the gas in the pipes is proportional to an integration value of (Qin-Qout), where Qin and Qout are defined as the quantity of solar energy and the quantity of heat-transfer to the gas respectively, the integration value should be 0 so that T may be equal to Tset which is dependent upon the ability and/or rating of the Stirling engine.
(2) Since a pressure P of the gas is proportional to the quantity of heat-transfer to the gas, a linear region corresponding to Qin=Qout is obtained when P is set to Qout for example. Thus, an output region of the Stirling engine which depends on the minimum or idle pressure and the maximum pressure, results in the determination of the maximum value and the minimum value of Qin. Therefore, the operating range of the engine can be determined.
Consequently, the pressure P can be controlled in response to the variation of Qin within the operating range derived from the above-items (1) and (2), thereby keeping the equivalence of Qin and Qout or the equation Qin=Qout. In light of this fact, in the conventional Stirling engine, the variable temperature T of the gas in the pipe according to the variation of Qin is detected, the difference between T and Tset is calculated, and the pressure P is varied according to the resulting difference to thereby keep the temperature of the gas at a set value. Thus, the output of the engine is maintained at a set value.
However, since solar radiation varies abruptly in magnitude, the temperature of the gas in the pipe varies in a similar manner. Thus, to maintain the temperature of the pipe at a set value, the pressure of the gas must be quickly adjusted in a precise manner. However, since the control of the pressure of the gas cannot be easily performed, the engine output becomes unstable. Further, in order to control the pressure of the gas in that manner, the valves for increasing the pressure and for reducing the pressure should have a special construction that permits the valves to be controlled in a precise manner and which permits a large amount of gas to be passed therethrough. However, the construction of such a valve can be very expensive.